1. Field of the Invention
The present invention relates to a shielded metal arc welding machine, and particularly relates to a shielded metal arc welding machine having both a constant current output characteristic and a drooping output characteristic having an adjustable drooping degree and showing a characteristic easily used by skilled and unskilled workers.
2. Description of the Related Art
Conventionally, an output characteristic of a welding machine is given a constant current output characteristic to enable an unskilled worker to perform stable arc welding. However, there exists a problem difficult to address with only the constant current output characteristic, depending on the kinds of welding rods and welding positions, and there is provided a welding machine in which a drooping output characteristic is combined with a constant current output characteristic, and a transition point of the characteristics is made variable (see Japanese Patent Laid-Open No. 11-170046).
This includes a constant current output characteristic I in a welding current region, a constant current output characteristic III for maximum current control, and a drooping output characteristic II connecting these two constant current output characteristics I and III, as shown in FIG. 6. Since the constant current output characteristic which is normally used and the drooping output characteristic are optionally connected and combined, welding operations corresponding to various welding rods can be performed.
The drooping output characteristic II which changes a drooping characteristic transition voltage as shown in FIG. 7A and the drooping output characteristic II which changes a constant current characteristic transition current as shown in FIG. 7B are provided.
However, the above described welding machine which enables unskilled workers to perform stable arc welding is not always suitable for skilled workers to exhibit their skill. Skilled workers have the experience of performing welding operations while performing fine regulation of heat input by shifting the intersections of a drooping output characteristic line and welding load characteristic lines L, L′ and L″ by using a direct-current generator (third blush generator) having the drooping output characteristic as shown in FIG. 8 to adjust the welding current to be I, I′ and II″.
On the other hand, in the welding machine having the constant current output characteristic, when the transition point to the drooping output characteristic from the constant current output characteristic is set at the welding load characteristic line L as shown in FIG. 9, the welding current can be regulated in the output current region larger than that at the transition point, but in the smaller current region, the current becomes a constant current and cannot be regulated.
The present invention is made in view of the above described point, and has an object to provide a shielded metal arc welding machine having a characteristic with a constant current output characteristic and a drooping output characteristic properly combined.